One motivation for making metallic-based nanoparticles is the novel performance achieved at the nano-scale relative to bulk materials. Materials of nanoscopic dimensions offer a variety of different properties than those observed on the macroscale, thus potentially enabling a variety of unique applications. In particular, nanometals exhibit a variety of electronic, optical, magnetic and/or chemical properties which are typically not achievable when metallic materials are in their bulk form. For example, metals that are relatively inert at the macroscale, such as platinum and gold, are excellent catalysts at the nanoscale. Further, combinations of two different metals (bi-metallic) at the nanoscale offer further intriguing performance issues. The different metals may result in mixtures of metals, alloys or heterogeneous structures, each of which my exhibit different physical properties and/or performance characteristics. Applications for bi-metallic nanoparticulate metals include electronics and computing devices, bionanotechnology, medical treatment and diagnosis and energy generation and storage. The use of these bi-metallic nanometals for a variety of applications requires efficient and safe approaches for manufacturing such materials.
In general, two fundamentally different approaches have been used to manufacture bi-metallic nanomaterials and they are referred to as “top-down” and “bottom-up” approaches. In the top-down approach, bi-metallic nanomaterials are manufactured from larger entities typically, without atomic-level control. Typical top-down approaches include such techniques as photolithography and electron-beam lithography which start with large materials and use either machining or etching techniques to make small materials. Laser ablation is also a known top-down approach.
In contrast, in the “bottom-up” approach, bi-metallic nanomaterials are manufactured from two or more molecular components which are caused to be assembled into bi-metallic nanoparticulate materials. In this regard, building blocks are first formed and then the building blocks are assembled into a final nano-material. In the bottom-up approach, there are a variety of general synthetic approaches that have been utilized. For example, several bi-metallic approaches include templating, chemical synthesis, sonochemical approaches, electrochemical approaches, sonoelectrochemical approaches, thermal and photochemical reduction methods including γ-ray, x-ray, laser and microwave, each of which has certain negative process and/or product limitations associated therewith.
Whichever approach is utilized, results of bi-metallic particle size control, particle size distribution, shape control, configuration or structure control, ability to scale up, and compatibility of the formed bi-metallic nanomaterial in the ultimate application, are all issues to be considered.
In the case where two metals are formed into bi-metallic nanoparticles, further considerations such as whether the bi-metallic nanoparticles are alloys, partial alloys or partially phase segregated or completely phase segregated are also important because the specific configuration of the nanoparticles can result in different performance (e.g., biologic or catalytic). A variety of techniques exist for forming two different metals into a variety of bi-metallic nanoparticles, some of which are discussed below.